Very large scale integrated circuit devices typically are manufactured on a substrate, such as a silicon wafer, by a sequence of material additions, such as low pressure chemical vapor depositions, sputtering operations, among others; material removals, such as wet etches, reactive ion etches, among others; and material modifications, such as oxidations, ion implants, among others. Typically, these physical and chemical operations interact with the entire substrate. For example, if a substrate is placed into an acid bath, the entire surface of the substrate will be etched away. In order to build very small electrically active devices on a substrate, the impact of these operations has to be confined to small, well-defined, regions.
Lithography in the context of VLSI manufacturing includes the process of patterning openings in photosensitive polymers, sometimes referred to as "photoresists" or "resists", which define small areas in which substrate material is modified by a specific operation in a sequence of processing steps. The manufacturing of VLSI chips typically involves the repeated patterning of photoresists, followed by etch, implant, deposition, or other operation, and ending with the removal of the exposed photoresist to make way for the new photoresist to be applied for another iteration of this process sequence.
Basic lithography systems typically include a source of light, typically not visible light, a stencil or photomask including a pattern to be transferred to a substrate, a collection of lenses, and a means for aligning existing patterns on the substrate with patterns on the mask or stencil. The aligning may take place in an aligning step or steps and may be carried out with an aligning apparatus. Typically, chips may be processed on a wafer including from 50 to 100 chips. The wafers may be patterned in steps of one to four chips at a time. As a result, such lithography tools are commonly referred to as "steppers".
The resolution of optical projection systems such as lithography steppers is limited by the wavelength, the numerical aperture of the projection optics used in the system, and a constant related to how well a combined lithography system can utilize the theoretical resolution limit in practice. The highest resolution in optical lithography is currently achieved with deep ultraviolet (DUV) steppers operating at 248 nm wavelengths. Mid-ultraviolet (MUV) steppers with a wavelength of 356 nm are also in widespread use.
Conventional photomasks typically consist of chromium patterns on a quartz plate, allowing light to pass wherever the chromium has been removed from the mask. Light of a specific wavelength is projected through the mask onto the photoresist coated substrate, exposing the photoresist wherever chromium has been removed from the mask permitting light to pass through the mask. Exposing the resist to light of the appropriate wavelength causes modifications in the molecular structure of the resist polymers, which permits the use of developer to dissolve and remove the resist in the exposed areas. Resists that act as just described are known as "positive" resists. On the other hand, negative resist systems permit only unexposed areas to be removed by the developer.
Photomasks, when illuminated, can be pictured as an array of individual, infinitely small light sources that can be either turned on, such as areas not covered by chromium or other material, or turned off, such as areas covered by chrome or other material. If the amplitude of the electric field vector that describes the light radiated by these individual light sources is mapped across a cross-section of the mask, a step function will be plotted reflecting the two possible states that each point of the mask can be found, either light on or light off.
Conventional photomasks are commonly referred to as "chrome on glass" (COG) binary masks, due to the binary nature of the image amplitude. The perfectly square step function of binary masks actually exists only in the theoretical level of the exact mask plane. Any distance away from the mask, such as at the substrate plane, diffraction will cause images to exhibit a finite image slope. At small dimensions, that is, when the size and spacing of the images to be printed are small relative to the wavelength and inverse of the numerical aperture, the electric field vectors of adjacent images will interact and add constructively.
Therefore, not only is diffraction a phenomenon that must be addressed when dealing with very small images, interference must also be addressed. The resulting light intensity curve between features is not completely dark, as a result of the diffraction and interference phenomenon. Rather, the light intensity curve exhibits significant amounts of light intensity created by the interaction of adjacent features.
The resolution of an exposure system is limited by the contrast of the projected image, that is, the intensity difference between adjacent light and dark features. An increase in the light intensity in nominally dark regions will eventually cause adjacent features to print as one combined structure rather than as discrete images.
The quality with which small images can be replicated in lithography depends largely on the available process latitude, that is, the amount of allowable dose and focus variation that still results in the correct image size formation. Phase shift mask (PSM) lithography improves the lithographic process latitude by introducing a third parameter on the mask. The third parameter is the electric field vector associated with the light produced by the light source.
The electric field vector, like any vector defined quantity, has a magnitude and direction. Therefore, in addition to turning the electric field amplitude on and off, the electric field vector can be turned on with a phase of about 0.degree., or with a phase of about 180.degree.. This phase variation is achieved in phase shift masks by modifying the length that a light beam travels through the mask material. By recessing the mask, such as by etching, to an appropriate depth, light traversing the center portion of the mask and light traversing the thinner portion of the mask may be made to be about 180.degree. out of phase. Alternatively, a material, such as a transparent thin film, may be applied to the surface of the mask to accomplish the same difference in traverse distance.
A 180.degree. phase difference means that the electric field vectors of light traversing adjacent portions of the mask should be of about equal magnitude but in substantially opposite directions. As a result, any interaction between light beams passing through the adjacent areas results in substantially perfect cancellation. Greater detail regarding phase shift masks is provided by "Phase-shifting Mask Strategies: Isolated Dark Lines", Mark D. Levinson, Microlithography World, March/April 1992, pp. 6-12, the entire contents of which are hereby incorporated by reference.
As can be appreciated from the above discussion, it is important to verify the nature of the shifting caused by the phase shift mask. In other words, it is important to measure the magnitude and direction of the phase shift. Currently, phase angle can be measured using dedicated interferometers. FIG. 1 shows an example of an apparatus that may be utilized in measuring phase angle on a phase shift mask.
Utilization of an apparatus such as that shown in FIG. 1 is described, for example, in "Direct Phase Measurements in Phase Shift Masks", A. P. Ghosh and D. B. Dove, SPIE, Vol. 1673, Integrated Circuit Metrology, Inspection, and Process Control VI (1992); "Interferometer for Phase Measurements in Phase Shift Masks", Derek B. Dove, Trieu C. Chieu, and Amal P. Ghosh, SPIE, Vol. 1809, 12th Annual BACUS Symposium (1992); "A New Tool for Phase Shift Mask Evaluation, the Stepper Equivalent Aerial Image Measurement System--AIMS.TM.", Russell A. Budd, John Staples, and Derek Dove, SPIE, Vol. 2087, Photomask Technology and Management (1993), "Accurate Phase Measurement in Phase-Shift Masks with a Differential Heterodyne Interferometer", Hiroshi Fujita, et al., Proceedings of the 1994 IEEE Instrumentation and Measurement Technology Conference, Part 2, IMTC May 10-12, 1994 Hamamatsu, pp. 683-688; "Measurement of Phase-Shift Masks", F. C. Chang, et al., SPIE, Vol 1926, pp. 464-471, 1993; "Impact of Attenuated Mask Topography on Lithographic Performance", Richard A. Ferguson, et al., SPIE, Vol. 2197, pp. 130-139, 1994; "A New Mask Evaluation Tool, the Microlithography Simulation Microscope Aerial Image Measurement System", R. A. Budd, et al., SPIE, Vol. 2197, pp. 530-540, 1994; "Application of an Aerial Image Measurement System to Mask Fabrication and Analysis", Richard A. Ferguson, et al., SPIE, Vol. 2087, Photomask Technology and Management (1993), pp. 131-144; "Application of the Aerial Image Measurement System (AIMS.TM.) to the Analysis of Binary Mask Imaging and Resolution Enhancement Techniques," R. Martino, et al., SPIE, Vol. 2197, pp. 573-584, 1994; and "Repair System for Phase Shift Masks," D. B. Dove and A. Ghosh, IBM Technical Disclosure Bulletin, Vol. 36, No. 05, May 1993, pp. 279-280; the entire disclosures of all of the above are hereby incorporated by reference.
Measurement of the actual phase shift typically is necessary to ensure that the physical structure created on the mask to cause the phase shift corresponds to the actual image that is desired to be projected upon the photoresist. Such verification is also necessary to ensure that the proper exposure is applied to the photoresist. Additionally, currently, phase shift masks are typically developed by computer programs that permit the image projected on the photoresist to be calculated from the above-discussed variables. The actual fabricated masks may contain small errors that may not be recognized or may be difficult to model. Hence, the reason for developing apparatus and method to measure the actual phase shift.